1. Field of the Invention
The present invention relates generally to spectrophotometric techniques for analyzing the content of a given fluid and, more particularly, to an apparatus for optimizing the sensitivity of such analysis. Even more particularly, the present invention relates to a multi-pass cell useful in absorption spectroscopy.
2. Related Art
Spectroscopy systems and methods are used to analyze the composition of various types of materials, including gases, liquids and the like. Spectroscopy is based on the fact that different chemical compositions absorb energy, e.g., light, at different frequencies, so that measuring the frequency of light passed through a sample can be used to identify which light frequencies were absorbed by the sample and which were not. Thus, the chemicals present in the sample can be readily identified. Spectroscopy systems and methods also can identify the amount of light absorbed by the sample at each given frequency. Thus, the quantity of each chemical present in the chemical composition can be determined. In addition, such analysis can be performed with any one of various different ranges of light such as infrared, ultraviolet and the like, each of which pertains to a separate range of frequencies.
An absorption cell (or resonator) for holding the gas or liquid sample through which light is passed is used to perform spectroscopy analysis in conjunction with suitable spectroscopy equipment, such as a light emitter and a light detector. It has long been realized that to increase the sensitivity in providing both quantitative and qualitative analyses, the light must be passed through a very large percentage of the available sample. Thus, absorption cells have been provided with “folded” light paths, in which mirrors reflect the light back and forth within the cell, such that the light makes multiple passes through the sample. The folded light path increases the optical path length between the light emitter and the light detector to thereby increase the sensitivity of a spectroscopy system incorporating an absorption cell producing a folded light path. Examples of existing “multi-pass” absorption cells are shown in U.S. Pat. Nos. 4,322,621; 4,749,276; 5,220,402; 5,440,143; 5,459,566; 5,485,276; 5,714,759; 5,731,583; and 5,949,537.
While many of the existing “multi-pass” absorption cells are effective in increasing the optical path length without greatly increasing the size or volume of the sample, the throughput (etendue) of these cells can be relatively small. The small throughput limits the amount of light that can be directed through the sample and limits the sensitivity of the resulting measurement.
What is still desire is an improved “multi-pass” absorption cell that causes light to pass through a very large percentage of a sample contained in the cell. Preferably, the improved absorption cell will prevent the loss of significant amounts of light through ends of the cell and increase the throughput of the cell. In addition, the improved cell will preferably be compact, robust, and easier to manufacture.